The purpose of a lamp is to convert electrical energy to visible light. There are a variety of lamps used in the lighting industry. Some examples are high intensity discharge (“HID”), fluorescent, incandescent, and light emitting diode (“LED”). Each of these lamps emits and dissipates energy in the form of radiant energy and heat in various amounts. For example, a 400 watt metal halide lamp converts approximately 112 watts to visible energy, 20 watts to UV energy, 72 watts to IR energy, while the remaining 200 watts of energy is converted to heat and dissipated to the surrounding environment via conduction through the lamp base and convection off the glass envelope. An LED used for lighting or illumination converts electrical energy to light in a fundamentally different way than HID, fluorescent, and incandescent lamps, resulting in very little radiant energy outside the visible spectrum. The bulk of the energy lost in the conversion process is dissipated as thermal energy through the LED chip and the mechanical structure that surrounds it. The energy conversions (percent of electrical energy input) for the aforementioned light sources are shown in the Table 1.
TABLE 1Energy conversion of various light sources(percent of electrical energy input)HIDFluorescentIncandescentLEDVisible2823512UV5000IR1836900Total Radiant51599512Conduction &4941588Convection
As shown by Table 1, a significant amount of energy is converted to heat by the lamp. In any luminaire design, the heat generated by the lamp may cause problems related to the basic function of the lamp and luminaire. Benefits associated with effective removal of thermal energy from within the luminaire include improved luminaire life, smaller (lower cost) package sizes, and improved lumen output in some lamp types, such as fluorescent and LED. An additional benefit of removing heat from the luminaire is that the luminaire may then be operated in a higher ambient temperature environment without compromising luminaire life or performance. In the case of an LED, better thermal management allows the LED to be driven at higher power levels while mitigating the negative effects on life and light output normally associated with higher power input levels.
There are three mechanisms for dissipating thermal energy from an LED: conduction, convection, and radiation. Conduction occurs when LED chips, the mechanical structure of the LEDs, the LED mounting structure (such as printed circuit boards), and the luminaire housing are placed in physical contact with one another. Physical contact with the LEDs is generally optimized to provide electrical power and mechanical support. Traditional means of providing electrical and mechanical contact between LEDs and the luminaire provide poor means of conduction between the LEDs and external luminaire surfaces (such as die cast housing). In addition, the location of LEDs is often determined by the desired optical performance of the luminaire. This often necessitates mounting LEDs a large distance from effective heat dissipating structures of the luminaire, which further impedes the conductive transfer of heat out of the luminaire envelope by creating a longer thermal path, introducing additional thermal interfaces, introducing materials with a lower thermal conductivity, or a combination thereof. A further disadvantage of using a thermally conductive structure within the luminaire envelope is that it allows dissipation of heat into the enclosure, which is generally sealed. This effectively raises the ambient temperature of the air surrounding the LEDs, thus compounding thermal related failures.
Convection occurs at any surface exposed to air, but may be limited by the amount of air movement near the emitting surface, the surface area available for dissipation, and the difference between the temperature of the emitting surface and the surrounding air. In many cases, the luminaire is enclosed further restricting airflow around the LEDs. In such an enclosure, heat generated by the LEDs is transferred by convection to the air within the enclosure, but cannot escape the boundaries of the enclosure. Although the LED itself does not contribute significant amounts of heat due to its small size, the components that are used to mount the LEDs are often large, thus allowing greater dissipation to the air within the enclosure by convection. As a result, the air within the enclosure experiences a build up of heat, which elevates lamp and luminaire temperatures and may lead to heat related failures. For example, in luminaires with electronic ballasts and components, excessive heat can shorten the life of the electronic components, resulting in premature failure of the lighting system.
Radiation is the movement of energy from one point to another via electromagnetic propagation. Much of the radiant energy escapes the luminaire through the clear optical elements (light emitting zones, lenses, etc) and reflectors, which are designed to redirect the radiant energy (visible light in particular) out of the luminaire according to the needs of the application. The radiant energy that does not escape through the lenses is absorbed by the various materials within the luminaire and converted into heat.
Of these three modes of thermal transfer, providing an effective conduction path often allows the greatest amount of controlled heat removal from within a luminaire. This is especially pertinent for luminaires that are enclosed to meet the requirements of the application (weather-proofing, concealing electrical components, safety, etc). Of particular importance is the need to optimize the thermal path to allow a low thermal resistance from the LED heat source to the dissipating surface on the exterior of the luminaire, while minimizing the cross-sectional area of the thermal path along the interior of the luminaire enclosure. A heat pipe is one mechanism that has been used to remove heat under these conditions.
A heat pipe is a tube, usually comprised of metal, that is evacuated and sealed with a small amount of fluid inside. Because the tube is sealed and evacuated, the working fluid changes from liquid to vapor at a relatively low temperature compared to the boiling point of that fluid at normal atmospheric pressure. The choice of fluid and internal pressure determine the temperature at which vaporization occurs. When a heat source is applied, the fluid will vaporize and uniformly fill the tube, resulting in a state of equilibrium where the fluid exists in both liquid and vapor form based on the amount of heat applied. If there is a location on the tube wall that is cooler than the area where the heat source is applied, the vapor will condense at that location. When fluid changes state from vapor to liquid, large amounts of energy are released.
With the addition of a special structure inside the tube, called a capillary structure, the fluid in liquid form will readily return to the spot where the heat source is applied via capillary action. The addition of the capillary structure within the tube creates a double-phase change convective thermal transfer loop that achieves a high thermal transfer coefficient over relatively large distances and small cross-sectional areas compared to what can be achieved with other thermal transfer structures. A heat pipe thus allows a relatively small heat producing area to be coupled to a large heat-dissipating surface that is far away from the heat source using a relatively small cross-sectional area structure to couple to the heat source and transfer the heat to the larger dissipating region. Such an arrangement is advantageous when the heat source is located inside an enclosed cavity with limited surface area or complex geometry for coupling to and dissipating heat.
In addition to the issue of thermal management, two compounding challenges have limited widespread adoption of LEDs for general illumination: concern over availability of LEDs as the technology changes and the prohibitive expense associated with LED replacement. The concern over LED availability is due to the fact that LEDs are very new to the market within the historical perspective of HID and fluorescent light source availability. Because LED technology is new and rapidly developing, the form factor of individual LEDs and the efficacy of LEDs change on a yearly basis. LEDs that were introduced as little as five years ago are no longer available today. LEDs that were introduced a year ago have efficacy improvement of 20 to 50%. This means that an owner, performing the simple act of purchasing replacement LEDs, will have to reconsider the impact on light levels, type of optics used, LED drivers, and thermal performance of the system. Essentially, the owner is required to perform an entire re-evaluation of the lighting installation, which is a considerable expense. Alternatively, an owner may obtain purchase agreements with LED manufacturers that ensure future availability of LEDs as originally specified. This approach, however, defeats the future energy savings potential of efficacy improvements in LED technology. These considerations are the root causes of significant concern on the part of facility owners and operators when considering LED based lighting systems. Therefore, it is desirable to have a solution that allows for forward compatibility of LED changes without impact to the form factor, thermal, or optical performance of the luminaire.
As to the concern over the expense associated with LED replacement, it is generally accepted that properly designed LED light sources within luminaires will have a lifetime of 50,000 hours. This may seem like a long time to people unfamiliar with luminaire construction, or those accustomed to residential lighting systems. A lifetime of 50,000 hours, however, is not exceptional within the general lighting industry as HID and fluorescent light sources with typical lifetimes of 20,000 to 100,000 hours have been used for decades. Furthermore, while these light sources generally provide longer life, it is desirable that they are serviceable in the event of a failure because the installed lifetime of luminaires greatly exceed the lifetime of even a 100,000 hour light source, and thus the thermal path should be able to be engaged and disengaged in a highly repeatable method with minimal introduction of thermal resistances.
Accordingly, there is a need for an LED based lighting system that includes an optimized conduction path and dissipation area to significantly reduce the amount of heat transferred from the LEDs to the interior of the enclosure, thereby allowing LED luminaires to operate in a higher ambient temperature environment without compromising luminaire life or performance. Additionally, there is a need for LED based lighting systems that allow for forward compatibility of LED changes without impact to the form factor, thermal, or optical performance of the luminaire. Finally, there is a need for LED based lighting systems that provide for LED replacement with minimal introduction of thermal resistances into the thermal path by ensuring that the thermal path engages and disengages in a highly repeatable manner.